Photolithography is a lithographic technique used to transfer the design of circuit paths and electronic elements of a chip onto a wafer's surface. A photomask is created with the design for each layer of the board or wafer (chip). The board or wafer is coated with a light-sensitive film (photoresist) that is hardened when exposed to light shining through the photomask. The board or wafer is then exposed to an acid bath (wet processing) or hot ions (dry processing), and the unhardened areas are etched away.
Ideally, the photoresist pattern produced by the photolithography process and the substrate pattern produced by the subsequent etch process would precisely duplicate the pattern on the photomask. For a variety of reasons, however, the photoresist pattern remaining after the resist develop step may vary from the pattern of the photomask significantly.
For example, many of the integrated circuit elements formed on the wafer's surface comprise multiple layers of thin film, interferences from which can result in critical dimension (CD) variation. The interferences are mainly caused by the reflectivity from resist top and bottom interfaces, and can lead to the existence of standing waves and CD swing curves.
FIG. 1 is a cross-sectional view of an incident of a light beam shown on a film stack shown during lithography. The light 10 shines through the resist 12, which is also referred to as the ambient, to the film stack 14. A portion of the light 18 passes through the film stack 14 into the substrate 16, another portion of the light 20 is reflected from the surface of the stack 14, and yet another portion of light 22 is reflected at boundaries between the layers in the stack 14. Optical functions, such as reflectivity of layer boundaries, can be computed by a set of parameters that include light wavelength, thickness of the stack layers, and a complex index of refractivity for the ambient, substrate, and each film layer.
The optimization for the lithography application typically involves the minimization or maximization of reflectivity. The optimization can be carried out by finding values for the parameters that cause a cost function of reflectivity to yield an optimal value. It is difficult or very time consuming to obtain an optimal value for complex multilayer stacks 14.
Software programs are available that simulate lithography process steps and parameters. PROLITH™ by KLA-Tencor Corporation of San Jose, Calif. is an example of a standard lithography simulation program. PROLITH is capable of aerial image and three-dimensional resist image predictions. Although current lithography simulation programs, such as PROLITH, help lithographers reduce process development and process optimization times, such programs have limitations. PROLITH, for example, performs the reflectivity minimization calculation by fixing two variables and finding the minimum of the function using the remaining variables. Two other variables are fixed, and the process is repeated for each layer. Thus, using conventional lithography simulation programs is a very iterative and time consuming process. In addition, results are not usually very accurate.
Accordingly, what is needed is an improved method for obtaining an optimal reflectivity value for complex multilayer stacks. The present invention addresses such a need.